This application relies for priority upon Japanese Patent Application No. 10-71265, filed on Mar. 6, 1998, the contents of which are herein incorporated by reference in their entirety.
The present invention relates to a mask that may be used for selectively growing a solid over a substrate during the manufacture of a semiconductor device. The mask defines a region in which the solid is to be grown. The present invention also relates to a manufacturing method of such a mask, and a method for selectively growing a solid using the mask.
Selective growth of a solid is a common technology required during the manufacture of a semiconductor device, in which silicon (Si) or gallium arsenide (GaAs) is selectively grown on a defined region of a semiconductor substrate. Currently, a semiconductor such as Si or GaAs, a metal such as tungsten (W) or aluminum (Al); and a silicide compound have been used as a material for a thin film, which is capable of being grown selectively.
During a selective growth process, the surface of the substrate is covered with a mask so as to suppress the growth of a solid on an undefined region covered by the mask. The material for the mask is determined by taking into consideration the types of substrate and solid material to be grown. Typical materials for a mask includes silicon dioxide which is obtained through the thermal oxidation of silicon or a vapor deposition method, and silicon nitride, which is formed by the vapor deposition method. In general, the thickness of a mask is several tens of nanometers (nm) or more. Also, the region in which a solid is to be selectively grown is defined by depositing a photoresist over the mask and then patterning the mask by photolithography.
However, as semiconductor devices become smaller, a technology for reducing the size of the selective growth region from units of submicrons into units of nanometers is required together with an ultra thin mask for use in this process, having a thickness of several nanometers or less, e.g., below 10 nm. Also, as the thickness of the mask is reduced, a mask material with both reaction selectivity and stability is required. However, such thin film cannot be formed of a single chemical composition.
The above problem will be explained using a thin mask formed of silicon dioxide by way of example. Firstly, if the mask is thin, the mask will be susceptible to defects such as fine voids or holes, so that the selectivity will be deteriorated. Therefore, it is favorable to form such an ultra thin mask at a lower temperature than a conventional method so as to precisely control the thickness of the thin mask. However, these low temperature conditions can easily cause other defects in the mask. In order to reduce the density of defects in the mask, a high-temperature annealing is favorable. However, a silicon dioxide film with a thickness of several nanometers or less, e.g., below 10 nm, is thermally unstable on the silicon substrate. As a result, the silicon dioxide film decomposes and desorbs during any annealing step performed at approximately 800xc2x0 C. or more. Thus, it is not possible to anneal the silicon dioxide film at a high temperature.
A second problem occurs when the mask is used as an insulating layer in a final device. By using the phenomenon of a desorption of oxygen by irradiating electron beams onto the silicon dioxide film, a fine pattern having a thickness of several nanometers, e.g., below 10 nm, can be formed directly by irradiating electron beams onto the mask without using a resist. However, it is necessary to limit the desorption of oxygen from the mask within the surface layer, which is caused by the irradiation of electron beams, so as to use the mask as an insulating layer later. However, the general electron beams, which have an energy of 10 keV, because of their long free path of 10 nm or more within the solid, pass over the entire silicon dioxide with a thickness of several nanometers. In other words, the mask layer formed of only silicon dioxide cannot be used as an insulating layer if oxygen is desorbed from it by irradiating the electron beams.
To solve the above problems, it is an object of the present invention to provide a mask to be used for selectively growing a solid. It is also an object of the present invention to provide a manufacturing method for such a mask, in which the mask is formed out of an ultra thin film, the generation of defects is suppressed, and stability to both heat and further irradiation by electron beams is provided.
It is another object of the present invention to provide a method for selectively growing a solid on a substrate using this mask for the selective growth of a solid.
According to an aspect of the first object, there is provided a mask for allowing selective growth of a solid over a growth region of a substrate and suppressing growth of the solid over other regions of the substrate. This mask includes an underlayer formed over the substrate, and a surface layer formed over the underlayer, wherein the surface layer and the underlayer have different chemical compositions.
Preferably, the surface layer of the mask comprises silicon dioxide as its main component and the underlayer comprises silicon nitride as its main component. A portion of the surface layer over the growth region is preferably etched away. In addition, a portion of the underlayer over the growth region may also be etched away. The combined thickness of the underlayer and the surface layer is preferably 10 nanometers or less.
According to another aspect of the first object, there is provided a method for manufacturing a mask for selectively growing a solid over a growth region of a substrate and suppressing growth of the solid over other regions of the substrate. This method comprises: forming an underlayer comprising silicon nitride over the substrate, and forming a surface layer comprising silicon dioxide over the underlayer. Preferably, the combined thickness of the underlayer and the surface layer is 30 nanometers or less.
Also, there is provided a method for manufacturing a mask for selectively growing a solid over a growth region of a substrate and suppressing growth of the solid over other regions of the substrate. This method comprises: forming a surface layer comprising silicon dioxide over the substrate, and nitrifying the substrate underneath the surface layer. Preferably, the thickness of the surface layer is 10 nanometers or less.
To achieve the second object, there is provided a method for selective growth of a solid, comprising: forming a mask over a substrate for the selective growth of the solid, the mask comprising an underlayer and a surface layer, each of which has a different chemical composition, defining a selective growth region over the substrate, where the solid is to be grown, and selectively growing the solid in the selective growth region.
The underlayer preferably comprises silicon nitride and the surface layer preferably comprises silicon dioxide. The solid to be grown may comprise silicon, or it may comprise one of tungsten (W), aluminum (Al), or a silicide compound.
The step of defining the selective growth region may further comprise irradiating electron beams onto a portion of the surface layer over the growth region. The step of defining the selective growth region may further comprise: forming a photoresist pattern that exposes the selective growth region over the surface layer, and etching the surface layer using the photoresist pattern as an etching mask. The step of defining the selective growth region may even further comprise etching the exposed underlayer to expose the substrate, using the photoresist pattern as an etching mask.
The solid is preferably grown by one of a thermal CVD or a plasma CVD process, and the combined thickness of the underlayer and the surface layer is preferably 10 nanometers or less.
The present invention is obtained taking into consideration that the location of the growth region is determined by the surface layer of the mask and that the stability of the mask or the presence of defects such as voids are determined by the underlayer of the mask. In other words, a material with the optimal selectivity in growth is used for the surface layer and a material capable of improving the stability of the entire mask is used for the underlayer.
The mask for the selective growth of a solid is preferably in the form of a thin film over the surface of the substrate and preferably has at least a dual-layered structure. Any material that is capable of suppressing growth on the mask during the selective growth reaction of the solid may be used for the surface layer. The underlayer may be formed of any material that is stable to heat and excitation by electron beams or plasma, and that suppresses the structural defects, that has a good interfacial property or adhesiveness to the substrate. In other words, the underlayer may be formed out of a material that is capable of satisfying all the conditions required for the mask during the desired selective growth reaction.
Among the possible combinations of materials, a mask may be formed with a structure in which a silicon dioxide layer is formed over a silicon nitride layer. This mask can be widely applied for the selective growth.
With the surface layer formed of silicon dioxide, a silicon nitride underlayer is interposed between the substrate and the silicon dioxide surface layer. As a result, the problems of defective structure and weak stability to heat and electron beams resulting from a thinning of the mask can be eliminated.
Firstly, with regard to the defective structure, a combination of different materials, e.g., silicon nitride and silicon dioxide, and can suppress the generation of structural defects such as voids.
The thermal instability in the mask made of silicon dioxide only is caused by a decomposition between silicon as a substrate and the surface of the silicon dioxide layer. However, a decomposition does not occur between silicon and silicon nitride at a high temperature of 1000xc2x0 C. or more. Thus, the dual-layered structure having a silicon nitride underlayer beneath a silicon dioxide surface layer is thermally stable.
The problem of the weak stability to electron beams is solved by using silicon nitride with a chemical composition that has a strong stability to the electron beams irradiation. Also, the chemical properties of silicon dioxide forming a thin surface layer is substantially the same as those of silicon dioxide in the form of a general thick film or of a bulk type. Thus, silicon dioxide serving as a mask can suppress the growth of various solid materials thereon and a growth region can be defined in it by irradiating electron beams.
A mask with a surface layer formed of silicon dioxide and a silicon nitride underlayer interposed between the substrate and the surface layer of silicon dioxide can be manufactured by two methods. In a first method, the silicon nitride firm is formed preferentially. The silicon nitride layer is formed by a vapor deposition method or through nitrification of a substrate if a silicon is used as the substrate. Here, the thickness and the chemical composition of the silicon nitride layer are determined in consideration of the desired selective growth reaction or whether to use the mask in the final structure of the device such as an insulating layer, by controlling the conditions for vapor deposition or nitrification. The silicon dioxide layer formed on the silicon nitride layer is formed to be thicker than a single atomic layer by a vapor deposition method or by oxidizing the silicon nitride layer.
In a second method, the silicon dioxide layer is formed prior to the silicon nitride layer. In this case, the substrate is limited to a silicon substrate, and the silicon nitride layer is formed through nitrification between activated nitrogen that has been diffused through the silicon dioxide layer into the solid and the silicon of the substrate. In detail, the silicon dioxide layer is initially formed by the vapor deposition method or through oxidation of the silicon substrate, to a thickness which is as thin as 10 nm or less such that nitrogen can be supplied through the silicon dioxide layer. Here, exciting a nitrogen atmosphere by plasma facilitates the nitrification.
The effect of the mask with silicon dioxide and silicon nitride layers on the selective growth of a solid was certified by two selective growth methods including a thermal chemical vapor deposition (CVD) method using a silicon hydride gas and a plasma-excited CVD method using a silicon hydride gas and a hydrogen gas and periodically modulates the flowing amount of the silicon hydride gas for the growth.
Also, in the mask structure with the silicon dioxide and silicon nitride layers, it is possible to define a selective growth region by irradiating electron beams onto the surface layer of silicon dioxide. That the silicon nitride layer is stable to the irradiation of electron beams were verified by irradiating electron beams with a 15xcx9c25 keV energy up to 400C/cm2. In addition, the mask structure barely changed in the chemical composition after the annealing at 1000xc2x0 C. for 30 seconds, which shows stability at a high temperature.